The present invention relates to a thermal transfer recording material and particularly to a thermal transfer image-receiving sheet which can yield images with high dyeability, is free from heat fusing to a thermal transfer sheet at the time of image formation, and has satisfactory separability from the thermal transfer sheet.
Various thermal transfer methods are known in the art. One of them is a method wherein sublimation-transferable dyes are provided as recording agents and are thermally transferred from a thermal transfer sheet comprising a substrate sheet, such as a polyester film, bearing thereon these dyes onto an object colorable with a sublimable dye, for example, an image-receiving sheet comprising a receptive layer provided on paper, a plastic film or the like to form various full-color images.
In this case, a thermal head in a printer is used as heating means, and a large number of color dots of three or four colors with regulated heat quantity are transferred onto the image-receiving sheet by heating in a very short time, whereby full color of an original is reproduced by multicolor dots.
Since colorants used are dyes which are very vivid and highly transparent, the formed images have excellent reproduction of intermediate colors and gradation and have high quality which is equal to images produced by conventional offset printing and gravure printing and is comparable to the quality of full-color photographic images.
What is important for effectively carrying out the thermal transfer method is the construction of the thermal transfer sheet, as well as the construction of the image-receiving sheet on which an image is to be formed. Regarding conventional image-receiving sheets, for example, Japanese Patent Laid-Open Nos. 169370/1982, 207250/1982, and 25793/1985 disclose resins for the receptive layer. Specifically, vinyl resins, such as polyvinyl chloride resins, polyvinyl butyral resins, acrylic resins, cellulosic resins, olefin resins, polystyrene resins, polyester resins, polycarbonate resins and the like are disclosed as resins for the formation of the receptive layer.
In recent years, an improvement in printing speed (high-speed printing), which can shorten printout time per sheet, and power saving (low energy) printing, which can be driven by batteries for portable convenience, have become demanded. A receptive layer formed of a vinyl chloride-vinyl acetate copolymer resin is preferred as the receptive layer for high-speed printing and low-energy printing, because satisfactory density can be provided and, in addition, at the time of thermal transfer, abnormal transfer such as fusing does not occur between the thermal transfer sheet and the thermal transfer image-receiving sheet. Environmental problems, however, have led to a demand for a reduction in or total abolition of the use of vinyl chloride-containing materials. Further, other conventional thermal transfer image-receiving sheets and thermal transfer sheets disadvantageously cannot provide satisfactory print density.
The adoption of a method wherein the amount of dyes added to a binder for holding dyes in the thermal transfer sheet is increased, a method wherein a large amount of a plasticizer is added to the receptive layer, or a method wherein thermal transfer is carried out at high energy or low speed, is considered effective for providing satisfactory print density.
Increasing the amount of dyes, however, causes migration of the dye to the backside of the thermal transfer sheet. This disadvantageously causes a lowering in print density with the elapse of time, contamination of the backside, and contamination of a thermal head which shortens the service life of the thermal head. Further, at the time of thermal transfer, fusing occurs between the thermal transfer sheet and the thermal transfer image-receiving sheet probably due to plasticization of the dye binder by the dye.
The addition of a large amount of a plasticizer to the receptive layer softens the resin constituting the receptive layer and thus can improve dyeability, but on the other hand, poses problems including that mere contact of the receptive layer with the dye layer at room temperature causes dyeing of the receptive layer, a problem called xe2x80x9csmudge,xe2x80x9d i.e., unfavorable dyeing by waste heat generated in printing; fusing between the receptive layer and the dye binder in the thermal transfer sheet is likely to occur in a region from halftone region to high density region and, in this case, a large noisy sound is produced in the separation of the thermal transfer image-receiving sheet from the thermal transfer sheet at the time of printing, and, in some cases, the receptive layer is completely fused to the thermal transfer sheet, and, consequently, normal printing cannot be carried out, that is, abnormal transfer occurs.
Further, the addition of the plasticizer poses a problem of a change with the elapse of time, for example, that the formed image blurs with the elapse of time and the sensitivity in printing varies depending upon an environment in which the image-receiving sheet before the formation of an image is stored, making it impossible to provide prints having stable color tone. High-energy printing or low-speed printing is contrary to the demand in recent years, and, further, the thermal transfer at high energy causes fusing between the thermal transfer sheet and the thermal transfer image-receiving sheet at the time of thermal transfer and consequently causes abnormal transfer.
A method for solving the problem of the plasticizer is to adopt a multilayer structure in the receptive layer wherein a plasticizer-containing layer is provided as the lower layer (substrate side). In this case, however, the dyeability of the upper layer (surface layer) is so small that, in the case of direct printing, the dye cannot be diffused into the lower layer and, thus, the print density is low. Further, due to the multilayer structure, the production of the image-receiving sheet is complicated, and, thus, the production cost is disadvantageously high.
Accordingly, in a first aspect, an object of the present invention is to solve the above problems of the prior art and to provide a thermal transfer image-receiving sheet which has dyeability high enough to realize high-speed printing and low-energy printing, permits a protective layer to be thermally transferred onto the formed image, can avoid heat fusing between the thermal transfer image-receiving sheet and a thermal transfer sheet at the time of image formation, and has satisfactory separability from the thermal transfer sheet.
In general, what is important for effectively carrying out the formation of an image by thermal transfer is the construction of the thermal transfer sheet for feeding colorants, as well as the construction of the image-receiving sheet for receiving colorants for the formation of an image.
Regarding conventional image-receiving sheets, as described above, for example, Japanese Patent Laid-Open Nos. 169370/1982, 207250/1982, and 25793/1985 disclose resins for the receptive layer. Specifically, vinyl resins, such as polyvinyl chloride resins, polyvinyl butyral resins, acrylic resins, cellulosic resins, olefin resins, polystyrene resins, polyester resins, polycarbonate resins and the like are disclosed as resins for the formation of the receptive layer. Release agents usable in the image-receiving sheet include various silicone release agents, fluoro release agents, waxes, and surfactants.
In recent years, a method for image formation wherein, after image formation, a proper protective layer is provided according to purposes, has been mainly used from the viewpoints of improving storage stability of prints, such as lightfastness and chemical resistance, and providing added values of practicality, design, and security, such as the impartation of writing quality to the surface of prints and the formation of a hologram layer. For this reason, the image-receiving sheet should have satisfactory separability high enough to avoid heat fusing to the dye binder in the transfer sheet at the time of image formation. On the other hand, at the time of the transfer of a protective layer, the image formed face should have satisfactory adhesion to the protective layer. Thus, the image-receiving sheet should have contradictory properties.
Vinyl chloride-vinyl acetate copolymer resins have hitherto been extensively used as resins satisfying these properties. In recent years, however, environmental problems have led to a demand for a reduction in or total abolition of the use of vinyl chloride-containing materials. Also in this respect, the development of a novel resin for a receptive layer, which can satisfy both requirements, i.e., satisfactory separability from the thermal transfer sheet and good adhesion to the protective layer, has been demanded.
Accordingly, in a second aspect, an object of the present invention is to provide a thermal transfer image-receiving sheet, without use of any vinyl chloride resin, which can satisfy both requirements, i.e., satisfactory separability from the thermal transfer sheet at the time of image formation and good adhesion to the protective layer at the time of the transfer of a protective layer.
Thermal transfer recording materials, used with a thermal dye sublimation transfer method, comprising a thermal transfer sheet comprising a dye layer provided on a substrate sheet and a thermal transfer image-receiving sheet comprising a receptive layer provided on a substrate have hitherto been used. An increase in printing speed in thermal transfer printers in recent years, however, has posed a problem that conventional thermal transfer recording materials cannot provide satisfactory print density.
When the dye/resin (dye/binder) ratio in the dye layer of the thermal transfer sheet is increased for overcoming this problem, during the storage of the thermal transfer sheet in a rolled state, the dye is transferred onto the heat-resistant slip layer provided on the backside of the thermal transfer sheet. Upon rewinding of the thermal transfer sheet, the transferred dye is retransferred (kickbacked) onto other color dye layer or a transferable protective layer, and the thermal transfer of the contaminated layer onto the image-receiving sheet provides a hue different from a specified hue or causes the so-called xe2x80x9csmudge.xe2x80x9d
Further, when the thermal transfer printer is regulated to apply high energy at the time of thermal transfer in the image formation, the dye layer is fused to the receptive layer, resulting in the so-called xe2x80x9cabnormal transfer.xe2x80x9d The addition of a large amount of a release agent to the receptive layer for preventing the abnormal transfer lowers the print density.
Further, the thermal transfer sheet had the following problems. Specifically, it is said that, when the formed thermal transfer sheet is stored for a long period of time, the state of presence of dyes in the dye layer is changed, although this varies depending upon storage environment, and, consequently, the surface of the dye layer is brought to a dye-rich state. This change in the dye layer causes the dye to be easily transferred even at low energy. This poses a problem that printing using a thermal transfer sheet after storage for a long period of time after the production thereof is likely to cause a phenomenon wherein a higher density than desired is developed particularly in low density region, a phenomenon wherein the dye is disadvantageously transferred onto the image-receiving sheet by only the pressure applied by a platen at the time of printing, or a phenomenon wherein the dye is disadvantageously transferred by waste heat of the thermal head.
As described above, in order to cope with increased printing speed of the thermal transfer and to meet a demand for a higher level of properties of media, the regulation of the thermal transfer printer side and the modification of a thermal transfer recording material comprising a thermal transfer sheet and a thermal transfer image-receiving sheet have been made. These methods, however, have posed a problem of unsatisfactory printing density, contamination by kickback, or a change in print density during storage for a long period of time. Thus, prints having satisfactory quality could not have hitherto been produced.
Accordingly, in a third aspect, an object of the present invention is to provide a thermal transfer recording material which can cope with increased printing speed of the thermal transfer and can meet a demand for a higher level of properties of media and can yield high-quality prints.
The first invention relates to a thermal transfer image-receiving sheet comprising: a substrate sheet; and a receptive layer provided on at least one side of the substrate sheet, said receptive layer comprising at least a combination of a cellulose ester resin (A) having a degree of acetylation of 10 to 30% with a cellulose ester resin (B) having a degree of acetylation of less than 6%, the total degree of acetylation of the cellulose ester resin (A) and the cellulose ester resin (B) being 8 to 14%, the content of hydroxyl groups in the cellulose ester resin (A) and the content of hydroxyl groups in the cellulose ester resin (B) each being not more than 6% by weight, the remaining hydroxyl groups having been esterified with an organic acid excluding acetic acid.
The organic acid is preferably propionic acid and/or butyric acid.
Preferably, the receptive layer further comprises a compatible thermoplastic resin.
Preferably, the receptive layer comprises at least one plasticizer selected from the group consisting of phthalic acid plasticizers, phosphate plasticizers, polycaprolactones, and polyester plasticizers and the content of the plasticizer is not more than 15% by weight based on the total weight of the plasticizer and the resins constituting the receptive layer.
Preferably, the receptive layer comprises at least one release agent.
Preferably, the release agent comprises at least a modified silicone oil and/or a cured product thereof, a fluorosurfactant, and/or a silicone surfactant.
Preferably, the silicone surfactant is a polyether-modified silicone.
Preferably, after the formation of an image on the thermal transfer image-receiving sheet in its image-receiving face, a protective layer is transferred onto the image formed face.
In a thermal transfer image-receiving sheet comprising a substrate sheet and a receptive layer provided on at least one side of the substrate sheet, the receptive layer comprises at least a combination of a cellulose ester resin (A) having a degree of acetylation of 10 to 30% with a cellulose ester resin (B) having a degree of acetylation of less than 6%, the total degree of acetylation of the cellulose ester resin (A) and the cellulose ester resin (B) is 8 to 14%, the content of hydroxyl groups in the cellulose ester resin (A) and the content of hydroxyl groups in the cellulose ester resin (B) each are not more than 6% by weight, and the remaining hydroxyl groups has been esterified with an organic acid excluding acetic acid. By virtue of the above construction, a thermal transfer image-receiving sheet can be provided which can form an image using a non-polyvinyl choride material with high dyeability by high-speed printing and low-energy printing, has excellent separability from a thermal transfer sheet, is free from blurring derived from plasticizers, and can yield a thermally transferred image having storage stability.
Further, after the formation of an image on the thermal transfer image-receiving sheet in its image-receiving face, the transfer of a protective layer onto the image formed face can provide prints which have good fastness or resistance properties such as high lightfastness.
The second invention relates to a thermal transfer image-receiving sheet comprising: a substrate sheet; and a dye-receptive layer provided on at least one side of the substrate sheet, said dye-receptive layer containing, at least in its outermost surface portion, at least one polyether-modified silicone selected from the group consisting of polyether-modified silicones represented by formulae (B1), (B2), and (B3), said polyether-modified silicones having a siloxane content of 25 to 65% by weight: 
wherein polyether-modified silicones represented by formula (B1) are of grafting type, R represents H, an aryl group, or a straight-chain or branched alkyl group optionally substituted by a cycloalkyl group, m and n are each an integer of not more than 2000, and a and b are each an integer of 1 to 30; 
wherein polyether-modified silicones represented by formula (B2) are of end modification type, R represents H, an aryl group, or a straight-chain or branched alkyl group optionally substituted by a cycloalkyl group, m is an integer of not more than 2000, and a and b are each an integer of 1 to 30; and 
wherein polyether-modified silicones represented by formula (B3) are of main chain copolymerization type, R represents H, an aryl group, or a straight-chain or branched alkyl group optionally substituted by a cycloalkyl group, R1 represents an aryl group or a straight-chain or branched alkyl group optionally substituted by a cycloalkyl group, m and n are each an integer of not more than 2000, and a and b are each an integer of 1 to 30.
According to a preferred embodiment of the present invention, the weight ratio of ethylene oxide (EO) to propylene oxide (PO), EO/PO, in the polyether-modified silicones is 35/65 to 65/35.
According to a further preferred embodiment of the present invention, the polyether-modified silicone is contained in an amount of not more than 10% by weight based on 100 parts by weight of a resin component constituting the dye-receptive layer.
In the present invention, the dye-receptive layer may further comprise an epoxy-modified silicone and/or a methylstyrene-modified silicone.
Preferably, in the present invention, the resin component constituting the dye-receptive layer is a thermoplastic resin selected from the group consisting of acrylic resin, styrene resin, acryl-styrene resin, acrylonitrile-styrene resin, polycarbonate resin, cellulose ester resin, and mixtures of said resins.
Further, according to the present invention, there is provided an image formed object produced by forming an image on an image-receiving face of the above thermal transfer image-receiving sheet and then transferring a protective layer onto the image formed face.
The third invention relates to a thermal transfer recording material comprising: a thermal transfer sheet comprising a substrate sheet and a dye layer provided on at least one side of the substrate sheet; and a thermal transfer image-receiving sheet comprising a substrate and a receptive layer provided on at least one side of the substrate, a dye contained in the dye layer in the thermal transfer sheet being transferable onto the receptive layer in the thermal transfer image-receiving sheet by putting the thermal transfer sheet and the thermal transfer image-receiving sheet on top of each other, so that the dye layer faces the receptive layer, and heating the assembly by heating means, said dye layer comprising at least dyes and a binder resin, said dyes including at least two or more dyes having an identical basic skeleton, said dyes having an identical basic skeleton including at least one combination of dyes which are different from each other in melting point by 10xc2x0 C. or above, said receptive layer comprising a cellulose ester resin.
According to a preferred embodiment of the present invention, the dyes are yellow dyes having a basic skeleton selected from quinophthalone dyes represented by formula (C1) and dicyanostyryl dyes represented by formula (C2): 
wherein R1, R2, R3, R4, and R5 each independently represent a hydrogen atom, a halogen atom, a C1 to C8 alkyl group, a cycloalkyl group, an alkoxy group, an alkoxyalkyl group, an alkoxycarbonyl group, a thioalkoxy group, an alkylsulfonyl group, an amino group, a substituted or unsubstituted phenoxy group, or a substituted or unsubstituted thiophenoxy group, and R6 and R7 each independently represent a hydrogen atom, an alkyl group, an alkoxyalkyl group, a cycloalkyl group, an allyl group, an optionally substituted aryl group, an aralkyl group, a furfuryl group, a tetrahydrofurfuryl group, or a hydroxyalkyl group; and 
wherein R1 represents an allyl group or an alkyl group, R2 represents a substituted or unsubstituted alkyl group or an aryl group, A represents xe2x80x94CH2xe2x80x94, xe2x80x94CH2CH2xe2x80x94, xe2x80x94CH2CH2Oxe2x80x94, xe2x80x94CH2CH2OCH2xe2x80x94, or xe2x80x94CH2CH2OCH2CH2xe2x80x94, and R3 represents an alkyl group.
Further, according to a preferred embodiment of the present invention, the dyes are magenta dyes having a basic skeleton selected from imidazoleazo dyes represented by formula (C3) and anthraquinone dyes represented by formula (C4): 
wherein R represents an alkyl group, an alkenyl group, an aryl group, a cyanoalkyl group, or a substituted or unsubstituted alkoxycarbonylalkyl group, R1 and R2 represent an alkenyl group, an aralkyl group, or a substituted or unsubstituted alkyl group, X represents a hydrogen atom, a methyl group, a methoxy group, a formylamino group, an alkylcarbonylamino group, an alkylsulfonylamino group, or an alkoxycarbonylamino group, and Y represents a hydrogen atom, a methyl group, a methoxy group, or a halogen atom; and 
wherein R represents a hydrogen atom, a hydroxyl group, a substituted or unsubstituted alkyl group, or a substituted or unsubstituted alkoxy group, X and Y represent an amino group or a hydroxyl group, and n is 1 or 2.
According to a preferred embodiment of the present invention, the dyes are cyan dyes having a basic skeleton selected from indoaniline dyes represented by formula (C5) and anthraquinone dyes represented by formula (C6): 
wherein R1 represents a hydrogen atom; an alkyl group optionally substituted by a fluorine atom; an alkoxy group; an alkylamino group; an alkylcarbonylamino group optionally substituted by a fluorine atom; or a halogen atom; R2 represents a hydrogen atom; an alkyl group optionally substituted by a fluorine atom; an alkoxy group; or a halogen atom, R3 and R4 represent a hydrogen atom; an alkyl group optionally substituted by a fluorine atom; an alkoxy group; or a halogen atom, and R, R5, and R6 represent a hydrogen atom, a C1 to C6 substituted or unsubstituted alkyl group, an aryl group, or an alkoxy group; and 
wherein R1 and R2 represent a substituted or unsubstituted alkyl group, a substituted or unsubstituted aryl group, a substituted or unsubstituted allyl group, or a substituted or unsubstituted aralkyl group.
Further, according to a preferred embodiment of the present invention, the thermal transfer sheet comprises a yellow dye layer, a magenta dye layer, and a cyan dye layer provided in a face serial manner on the substrate sheet, the yellow dye layer comprises at least the above yellow dyes, the magenta dye layer comprises at least the above magenta dyes, and the cyan dye layer comprises at least the above cyan dyes.
According to a preferred embodiment of the present invention, the binder resin contained in the dye layer is any one of polyvinyl acetal resin and polyvinyl butyral resin.
According to a preferred embodiment of the present invention, the thermal transfer sheet comprises the dye layer and a transferable protective layer provided in a face serial manner on the substrate sheet.
According to a preferred embodiment of the present invention, in the thermal transfer image-receiving sheet, the receptive layer contains a thermoplastic resin compatible with the cellulose ester resin.
According to a preferred embodiment of the present invention, in the thermal transfer image-receiving sheet, the receptive layer contains not more than 15% by weight of at least one plasticizer selected from phthalic acid plasticizers, phosphate plasticizers, polycaprolactones, and polyester plasticizers.
According to the present invention, by virtue of a predetermined relationship between the basic skeleton and the melting point in dyes contained in the dye layer, a kickback phenomenon can be prevented, and the dyes are stably present. Further, the cellulose ester resin constituting the receptive layer can realize high print density and can impart good resistance to prints.